RAPID RUPTURE AND COMPLEX FAULTING OF THE MAY 12, 1990, SAKHALIN DEEPEARTHQUAKE - ANALYSIS OF REGIONAL AND TELESEISMIC BROAD-BAND DATA

A large (M(W)=7.1) deep earthquake (606 km) occurred beneath Sakhalin Island, about 200 km away from the nearest well-defined Wadati-Benioff zone in southwestern Kurile. Moment tensors obtained in various Frequ ency bands using the teleseismic data show that the Sakhalin event pro duced double-couple radiation over a very broadband frequency range, a nd the P waveforms appear simple at most teleseismic stations (DELTA=3 0-90-degrees). On the other hand, many broadband seismometers successf ully recorded the seismic waves at regional distances (DELTA=3-25-degr ees) in Japan, showing two significant arrivals in the P wave seismogr ams whose time interval is about 3 s. The two P wave arrivals have the same polarity, the same apparent velocity, and similar amplitudes, wh ich suggests that the two arrivals are the result of two subevents in the source process of the Sakhalin event. Analysis of the regional and teleseismic waveform data suggests an anomalously rapid slip propagat ion in the source process of the Sakhalin deep event. The difference i n P waveform between the regional and teleseismic stations can be expl ained by having a long distance (>14-15 km) and short time interval (< 1.5-2 s) between the two subevents, which implies that the apparent ru pture velocity is larger than the S wave velocity unless the two subev ents occurred simultaneously. The observation that the source duration of the Sakhalin event is shorter than those of other deep events of t he same size supports the notion of rapid slip-propagation for the Sak halin event.